Many tens of millions of gallons of high level liquid nuclear waste (supernate including water soluble salts) have accumulated over the past 28 years of operation at the Savannah River Plant, Department of Energy. (Other D.O.E. facilities have similar accumulations.) The radioactivity of the Savannah River supernate is about 4 Ci/gallon of which more than 99 percent is contributed by cesium-137. The remaining radioactivity is from strontium-90, ruthenium-106, plutonium and other isotopes. The supernate is stored in carbon steel waste tanks which are unacceptable for permanent storage. It is desirable to decontaminate the supernate and solidify it in a concrete material (saltcrete). The radioactive elements removed from the supernate should be combined with sludge from the waste tanks and advantageously solidified in borosilicate glass. A requirement for supernate decontamination is that a decontamination factor (DF) of greater than 10.sup.4 must be obtained, so that the decontaminated salt solution can be safely disposed of in the saltcrete form. Toward this end, an ion exchange process has been successfully demonstrated but it is rather expensive to build and operate. (See copending application of the common assignee: Ser. No. 375,232, filed May 5, 1982).
Sodium tetraphenylboron (NaTPB) is a well-known precipitating agent for gravimetric analyses of potassium, rubidium and cesium. H. Flaschka and A. J. Barnard, Jr., "Tetraphenylboron (TPB) As An Analytical Reagent"; Advances in Analytical Chemistry and Instrumentation, Volume 1, (1960), Sec. I-III, pp. 1-29. There have been many attempts to use NaTPB as a precipitation agent for cesium from dilute aqueous solution but the results were not encouraging. B. Kahn, D. K. Smith and C. P. Straub, "Determination of Radioactive Cesium in Water", Analytical Chemistry, Volume 29, (1957), p.1210, discarded the possibility of using NaTPB for this purpose because of the difficulty encountered in settling the cesium tetraphenylboron (CsTPB) precipitate from the aqueous liquid. T. H. Handley and C. L. Burros, "Determination of Radioactive Cesium", Analytical Chemistry, Volume 31 (1959), p. 332, found that it was impossible to filter or centrifuge the CsTPB in acetone, and, therefore, it was recrystallized by adding absolute alcohol. The CsTPB precipitate obtained from the alcoholic solution was easily filtered, but the overall chemical yield was only 75 percent, that is, the decontamination factor (DF).ltoreq.3.
U.S. Pat. No. 2,982,785, McKenzie, Pasco and Schulz, described a solvent extraction process in which NaTPB in hexone was used to remove cesium from aqueous solutions. The DF's for the study therein were 2 and 6 when the pH's were 2 and 13, respectively. H. Flaschka and A. J. Barnard, Jr., cited hereinabove, Section VII, pp. 69-77, in particular, at p. 74, when nitrobenzene was used as a solvent, obtained a DF of 8,800.+-.1400 over the pH range of 5 to 10. This is the highest DF reported in the known literature. Other shortcomings of these solvent exchange processes are: (1) a prolonged centrifugation was required to separate the phases because of emulsion formation; and (2) solvent extraction is intrinsically complicated.
Sodium titanate is known to have an affinity for strontium. It has been previously prepared in the form of an ion exchange material--U.S. Pat. No. 4,161,513. Sevald Forberg and Per-Inge Olsson. Forberg et al., however, does not show the use of sodium titanate slurry for ion exchange.